The invention relates to a method of winding a roll of sheet material and, in particular, to a method of providing a force to compensate the forces exerted on the shaft of the roll so that the roll can be formed uniformly.
This method and the associated apparatus are suitable, in particular, for roller assemblies used in the production of plastic film, coated (especially adhesive-coated) film and paper material. The material is wound onto a shaft to form a roll. The material can be fed from a feeder roller or passed between a contact roller and the roll as the roll is wound on the shaft.
In the case of a conventional central winder, the material is fed over a feeder roller to a roll that is disposed on a driven shaft to ensure that the material is guided to form a uniform roll. A gap is usually maintained between the feeder roller and the roll. A gap having a constant width can be maintained by continuously adjusting the position of the feeder roller as the roll diameter increases. Alternatively, the central winder can be configured so that the position of the shaft, instead of the feeder roller, is corrected.
In a surface or contact winder, a contact or winding roller is driven and pressed against the roll. Because the contact roller contacts the roll, the contact roller is driven to rotate in the opposite direction. The surface winder is less suitable for sensitive web or film material, but is more suitable for high winding speeds.
During the production of heavy or wide films, the shaft bends as the roll diameter increases due to the weight of the roll. The bending is dependent on the ratio between the weight of the roll and the length and the diameter of the shaft. In the case of a relatively small shaft diameter of, e.g., 74 mm, and a film or roll width of, e.g., at least 2000 mm, the bending is so great that no uniform line of contact over the entire roll width exists between the roll and the contact roller. Accordingly, the roll becomes undesirably misshapen and the risk of trapping air and forming folds in the wound layers of material increases. In the case of films coated with an adhesive, pockets of air trapped between the layers can cause the roll to be rejected.
In the case of winding devices with a contact roller, the contact roller and the roll contact one another. Accordingly, a large contact force causes further bending in the shaft. Consequently, the bending caused by the weight of the roll and the bending caused by the contact force produce a resultant bending in the shaft along a resultant direction. Therefore, a satisfactory roll cannot be formed.